10.1 E-R Relationships

	A relation in an E-R model is an association between two entities.
	For example, there could be a relationship Belongs_To between a Member entity and a Department entity in a library database.
	A distinction could be made between a relationship type and a relationship instance as illustrated in Figure 10.1.

Belongs_To is a relationship type where r1 is a relationship instance associating a member instance m1 with a department instance d2.

	Entities which are involved in a relationship are called relationship participants.
	The number of participants in a relationship is called the degree of the relationship.
	Thus, in the Belongs_To relationship,

		Member and Department are relationship participants.
		The degree of the relationship is two. A relationship can have attributes in similar way to entities.

10.1 E-R Relationships

10.1 E-R Relationships

	Entity participation in a relationship could be total or partial.
	Consider a relationship R that involves an entity E as a participant.

		If every instance of E participates in at least one instance of R, then E’s participation in R is said to be total.
		Otherwise, E participation in R is said to be partial.
		So, if each member must be part of a department, then the participation of the Member entity in the Belongs_To relationship is total.

The E-R model specifies another constraint on an entity’s participation in a relationship which is the Cardinality ratio.

		It specifies the number of relationship instances an entity can participate in.
		E.g., a Written_By relationship between a Book and Author entity could have a M:N cardinality ratio specifying that each book can be related to (written by) more than one author and an author can be related to more than one book.

	Figure 10.2 illustrates the diagrammatic representation of relationships in an E-R model.
	A relationship in an E-R model is represented by a diamond figure attached on both sides with participating entities.

10.1 E-R Relationships

	The cardinality ratio of a relationship is specified by attaching 1, M or N on each participating edge of the relationship diamond.
	The total participation of an entity in a relationship is represented by a double line connecting the entity with the relationship.
	A double diamond is used to represent an indentifying relationship associating a weak entity with the entity it depends on.

10.1 E-R Relationships